The present invention relates to an optical disk apparatus which records and reproduces information on an optical disk, and particularly to a laser driver which is useful for improving the quality of reproduced signal and to an optical disk apparatus which uses the laser driver.
Optical disk information reproduction apparatus based on the semiconductor laser adopts the high frequency superimposition scheme in which a high-frequency current is superimposed on a d.c. current for driving the semiconductor laser in order to reduce the laser noise which emerges due to the interference between the semiconductor laser and the reflected light from the recording medium. However, despite the use of the high frequency superimposition scheme, there was found a phenomenon of the presence of a significant residual laser noise. A study conducted recently revealed that this phenomenon is caused by the relaxation oscillation which arises when the semiconductor laser undergoes the high-frequency modulation such as the case of high frequency superimposition scheme, and that the resulting laser noise is dependent on the superimposed frequency. A method of setting the superimposed frequency in consideration of the laser noise caused by the relaxation oscillation is disclosed in Japanese Patent Unexamined Publication No. Hei 11-54826.
FIG. 7 shows a measurement result of the relation between the laser noise and the frequency and amplitude of the superimposed high-frequency current. The laser noise was managed to be within the allowable level by setting the superimposed current within region A or B on the graph of FIG. 7. However, the high frequency superimposing circuit has its operational efficiency falling as the frequency rises, resulting in an increased heat dissipation. The semiconductor laser operating at a higher temperature produces a larger laser noise in general, and therefore the region B located in the higher frequency range is not suitable for use. Another region C shown by hatching in FIG. 7 necessitates the reduction of unwanted radiation or electromagnetic interference (EMI) by means of an expensive shielding structure to meet the regulation, and therefore this region is avoided. On this account, for the fulfillment of both the reduction of laser noise and the reduction of EMI, the superimposed current needs to be set within the region A and at the same time outside the region C, i.e., the superimposed current must be within range D in terms of frequency and within range E in terms of amplitude.
FIG. 4 shows by block diagram a laser driver which controls the superimposed current. The laser driver is made up of a semiconductor laser drive circuit 1, an oscillator control circuit 3 and oscillator 4 which form a high frequency superimposing oscillator, and an adder 6. At information reproduction, the semiconductor laser drive circuit 1 releases a laser driving d.c. current 2 and the adder 6 superimposes the output (high-frequency current 5) of the oscillator 4 to the laser driving d.c. current 2, so that a resulting current drives a laser diode 8. The oscillator 4 has its output frequency and amplitude controlled by the oscillator control circuit 3, which is responsive to an oscillator control signal 31 provided by an external microcomputer. Based on this function, the prior art method sets the frequency and amplitude of the superimposed current to meet the above-mentioned conditions.